Light Mo Isotopes of Post-Collisional Ultrapotassic Rocks in Southern Tibet Derived From Subducted Indian Continental Crust

Recycling of molybdenum isotopes in continental subduction zones remains debated. In this contribution, we re-visit the Mo isotope compositions of the Sailipu post-collisional ultrapotassic rocks in the Himalaya-southern Tibet orogen. These ultrapotassic rocks have very varying & delta;98/95Mo values of -0.66 to -0.07 & PTSTHOUSND; and Mo/Ce ratios of 0.0008-0.005, which are lower than those of mid-ocean ridge basalts (MORB; & delta;98/95Mo = -0.20 & PLUSMN; 0.06 & PTSTHOUSND;, and Mo/Ce = 0.03) and oceanic subduction-related (i.e., mantle source involving fluids, residual slab, or oceanic sediments) magmatic rocks (e.g., modern arc lavas, Cenozoic OIB-type basalts in eastern China and the central Mariana Trough basalts in the back-arc basin, syn-collisional andesitic rocks in southern Tibet). Combined with the light Mo isotopes of the Himalayan schists and gneisses, we suggest that the light Mo isotopic signature of the Sailipu ultrapotassic rocks is derived from subducted Indian continental crust. This is consistent with the extremely low & delta;11B (-17.4 to -9.7 & PTSTHOUSND;) and B/Nb (0.16-1) values and enriched Sr-Nd-Pb isotopes of the Sailipu ultrapotassic rocks. Thus, this study reveals the recycling of light Mo-B isotopes during continental subduction and demonstrates that Mo-B isotopes can effectively distinguish between continental and oceanic subduction. Mo isotope systematics have been widely applied in the study of tracing recycled crustal materials, and abundant researches have proposed that heavy Mo isotopic compositions of arc-mafic magma can be ascribed to slab-dehydrated fluids. However, in continental subduction zones, the origin of the light Mo isotopes of post-collisional mafic rocks (oceanic sediments during prior oceanic subduction vs. subducted continental crust) remains controversial, hindering our understanding of the recycling of continental crustal materials. In this study, we report new Mo isotope data of post-collisional ultrapotassic rocks in the Lhasa block of the southern Tibetan plateau. We have used Mo isotope data along with B-Sr-Nd-Pb isotopes of these ultrapotassic rocks, in combination with Mo-B-Sr-Nd-Pb isotopes of the Himalayan crustal rocks (e.g., gneisses and schists) to trace the crustal components in the post-collisional mantle beneath southern Tibet. We concluded that the light Mo and B isotope compositions in southern Tibet were derived from subducted Indian continental crust rather than Neo-Tethyan oceanic sediments. Thus, this study not only reveals the recycling of light Mo-B isotopes in this typical collision orogen (i.e., Himalaya-Tibet orogen) but also shows the potential in discriminating between oceanic subduction metasomatism and continental subduction metasomatism. Post-collisional ultrapotassic rocks in southern Tibet have extremely light Mo and B isotope compositionsThese light Mo-B isotope features are derived from subducted Indian continental crustMo-B isotopes have the potential to discriminate between oceanic and continental subduction

Automated summary

The recycling of molybdenum isotopes in continental subduction zones is debated. This study re-visits the Mo isotope compositions of ultrapotassic rocks in the Himalaya-southern Tibet orogen and suggests that the light Mo isotopes in these rocks are derived from subducted Indian continental crust. The study highlights the potential of Mo-B isotopes in distinguishing between oceanic and continental subduction.